Add Gauge metric for BSL avaiability

[reasonerjt]

The label of the gauge is the name of BSL

Fixes #7039

Please indicate you've done the following:

• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file (make new-changelog) or comment /kind changelog-not-required on this PR.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

Attention: Patch coverage is 65.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 60.33%. Comparing base (28b2d11) to head (a550910).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/metrics/metrics.go	75.00%	4 Missing ⚠️
...g/controller/backup_storage_location_controller.go	33.33%	2 Missing ⚠️
pkg/cmd/server/server.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9059   +/-   ##
=======================================
  Coverage   60.33%   60.33%           
=======================================
  Files         378      378           
  Lines       42801    42821   +20     
=======================================
+ Hits        25823    25836   +13     
- Misses      15425    15432    +7     
  Partials     1553     1553           

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